I came across two papers on the website for Johannes Hanika, that are promising

Improved half vector space light transport

Abstract:"In this paper, we present improvements to half vector space light transport (HSLT) [KHD14], which make thisapproach more practical, robust for difficult input geometry, and faster. Our first contribution is the computationof half vector space ray differentials in a different domain than the original work. This enables a more uniformstratification over the image plane during Markov chain exploration. Furthermore, we introduce a new multichain perturbation in half vector space, which, if combined appropriately with half vector perturbation, makes themutation strategy both more robust to geometric configurations with fine displacements and faster due to reducednumber of ray casts. We provide and analyze the results of improved HSLT and discuss possible applications ofour new half vector ray differentials."

Abstract:"We present manifold next event estimation (MNEE), a specialised technique for Monte Carlo light transportsimulation to render refractive caustics by connecting surfaces to light sources (next event estimation) acrosstransmissive interfaces. We employ correlated sampling by means of a perturbation strategy to explore all halfvectors in the case of rough transmission while remaining outside of the context of Markov chain Monte Carlo,improving temporal stability. MNEE builds on differential geometry and manifold walks. It is very lightweight inits memory requirements, as it does not use light caching methods such as photon maps or importance samplingrecords. The method integrates seamlessly with existing Monte Carlo estimators via multiple importance sampling."